1. Field of the Invention
The present invention generally relates to a solid-state memory and a semiconductor device including a recording layer in which a phase transformation is made between a crystal state and an amorphous state. The recording layer of the present invention has a resistance difference between the crystal state and the amorphous state that can be utilized in recording and erasing of data. More specifically, the present invention relates to a phase-change random access memory, hereinafter referred to as a PRAM.
2. Description of Related Art
A PRAM performs recording and erasing of data by a change in physical properties of a recording layer. The change is caused by a primary phase transformation of the recording layer. The primary phase transformation is made between a crystal state and an amorphous state of the recording layer. Typically, the recording layer may be a Te-containing chalcogen compound. The PRAM has been designed based on those fundamental principles. The PRAM is described in Japanese Unexamined Patent Application, First Publication, No. 2002-203392, in p. 114 of “Technology and Materials for Future Optical Memories,” by Masahiro Okuda, CMC Publishing Co., Ltd., Jan. 31, 2004, and in p. 209 of “Basics and Applications of Optical Disk Storage,” by Yoshito Tsunoda et al., The Institute of Electronics, Information and Communication Engineers, 1995.
The recording layer that is used for the PRAM that performs recording and erasing of data can generally be formed by utilizing a vacuum film formation method such as a sputtering method between electrodes. In general, the recording layer can be realized by a single layered structure of an alloy thin film that is formed by using a compound target. The thickness of the alloy thin film is between 20 nm and 50 nm, and the alloy thin film is realized not by a single crystal but by a polycrystal.
There has been performed analyzation of the crystal structure and amorphous structure of the Te-containing chalcogen compound by using an X-ray since the second half of 1980. A Te atom and a Sb atom included in the Te-containing chalcogen compound have adjacent atomic numbers. The electron numbers of the Te atom and the Sb atom are only different by one. It is difficult to distinguish the difference using an X-ray diffraction method or an electron beam diffraction method. The detailed structure of the Te-containing chalcogen compound was unknown until 2004.
The chalcogen compound including Ge, Sb and Te has been realized as the recording layer which, for example, can be used as an optical recording medium. GeSbTe (225) and a compound similar to GeTe—Sb2Te3 (225, 147, 125) are examples of the chalcogen compound that are experimentally known as having good properties. The crystal structure of the chalcogen compound is similar to the simple cubic lattice of NaCl. Te atom is at a site corresponding to Na of the NaCl structure, and Ge atom or Sb atom is at a site corresponding to Cl of the NaCl structure. The atomic arrangement in the chalcogen compound was thought to be random. Those are described in p. 7020-7028 of “Structure of Laser-Crystallized Ge2Sb2+xTe5 Sputtered Thin Films for Use in Optical Memory,” by Yamada, T. Matsunaga, Journal of Applied Physics, 88, 2000, for example.
There has been performed analyzation of the crystal structure of the Ge—Sb—Te compound using a synchrotron radiation facility. The analyzation is described in “Understanding the phase-change mechanism of rewritable optical media” by A. V. Kolobov et al., Nature Materials 3, 703, 2004.
A typical result of the above analyzation will be described. FIG. 2 is a view of the crystal state of the Ge—Sb—Te compound. The crystal of the Ge—Sb—Te compound is constituted of Te atoms 1, Sb atoms 2, and Ge atoms 3. The crystal structure of the Ge—Sb—Te compound is similar to the simple cubic lattice of NaCl. The Te atoms 1 are at the site corresponding to Na of the NaCl structure, and the Sb atoms 2 or the Ge atoms 3 are at the site corresponding to Cl of the NaCl structure. The atomic arrangement in the crystal state of the Ge—Sb—Te compound is not random. The crystal structure of the Ge—Sb—Te compound is distorted.
FIG. 3 is a view of the amorphous state of the Ge—Sb—Te compound. The atomic arrangement in the amorphous state of the Ge—Sb—Te compound is not random. The configuration of the amorphous state of the Ge—Sb—Te compound is distorted a little maintaining the configuration. In the crystal state, the Ge atoms 3 shift by 2 angstrom toward the Te atoms 1. But the crystal structure is slightly shifted, while maintaining the unit of the crystal structure. By the shifting of the Ge atoms 3, the Ge—Sb—Te compound slightly comes toward ferroelectric states.
FIG. 4A is another view of the crystal state of the Ge—Sb—Te compound corresponding the state of FIG. 2. FIG. 4B is another view of the amorphous state of the Ge—Sb—Te compound corresponding to the state of FIG. 3. High speed switching operation is performed between the crystal state of FIG. 4A and the amorphous state of FIG. 4B. The high speed switching operation is performed repeatedly by restoring the distorted structure.
The current used in recording and erasing of data in a solid memory needs to be decreased. The number of the times recording and erasing of data can be repeated needs to be increased. A main limiting factor to the number of the times recording and erasing of data can be repeated is heat flow of a recording film at high temperatures or a subsequent transition of the recording film. This analyzation is described on p. 209 of “Basics and Applications of Optical Disk Storage,” by Yoshito Tsunoda et al., The Institute of Electronics, Information and Communication Engineers, 1995, for example.